1. Technical Field of the Invention
The present invention generally relates to optical amplifier circuits. More particularly, and not by way of any limitation, the present invention is directed to method and apparatus for implementing an optical supervisory channel in connection with such circuits using broadband noise modulation.
2. Description of Related Art
The laying of new fiber was once the only way to cope with fiber exhaust in optical telecommunications networks. In addition to being labor- and cost-intensive, this “solution” did not enable network operators to provide additional services to customers. In the early 1980s, time-domain multiplexing (“TDM”) technology enabled an increase in the bit rate of optical telecommunications networks. With TDM, the capacity of a single fiber was increased by dividing time into small intervals and multiplexing the various signals onto these separate time intervals.
In TDM systems, each optical fiber is capable of transporting an optical signal from a single laser. The optical signal is converted into an electrical signal, electrically reshaped, retimed, and reamplified (“3R regenerated”), and finally transformed back into an optical signal, resulting in additional losses. Wavelength-division multiplexing (“WDM”) networks, which enabled the simultaneous transmission of multiple signals of different wavelengths over a single fiber, were deployed in the late 1980s and proved in many cases to be a preferable alternative to TDM.
During the 1990s, WDM networks were developed that enabled up to four different signals to be transmitted over one fiber at different wavelengths within the same optical window. For obvious reasons, this type of WDM network necessitates the use of narrow lasers.
In order to increase the number of services that can be provided, the channel spaces can be moved closer together, creating Dense WDM (“DWDM”). This technology economically increases transport capacity through the utilization of existing fiber routes and terminal equipment.
A DWDM system can be described as a parallel set of optical channels each using a slightly different wavelength, but all sharing a single transmission medium or fiber. In a typical embodiment, various signals are fed to optical transmission modules. The optical output signals are converted to defined wavelengths within a window around a specific wavelength, e.g., 1550 nanometers (“nm”), via suitable wavelength transponders. An optical DWDM coupler then multiplexes these optical signals onto a single fiber and forwards them to an optical fiber amplifier (“OFA”). OFAs work solely in the optical domain, meaning that an optical signal can be amplified without converting the signal to an electrical signal prior to amplification and converting the amplified electrical signal back to an optical signal. Moreover, OFAs perform a 1R (i.e., optical reamplification) regeneration, as apposed to a 3R regeneration, as discussed above, and simultaneously amplify each wavelength of a DWDM signal without requiring the signal to be demultiplexed before and then remultiplexed after amplification. A major advantage of OFAs is their transparency to signal speed and data type.
One of the more common types of OFAs currently in use is the Erbium-Doped Fiber Amplifier (“EDFA”), which comprises a section of optical fiber doped with erbium ions. Radiation from a pump laser outside the data wavelength range is coupled into the fiber to amplify the data signal. Specifically, in one embodiment, a data signal input to the OFA is provided to a wavelength combiner via an optical isolator, a function of which is to attenuate reflections. An input signal detector after the optical isolator detects the level of the input signal and generates an electrical signal corresponding to this level. A pump laser supplies a second input to the wavelength combiner. The pump laser produces light whose wavelength is e.g., 980 or 1480 nm, whereas the wavelength of the light of the signal to be amplified is 1550 nm, for example. The photons of the pump laser are conducted to the erbium-doped fiber where they excite erbium atoms of the fiber. Some of the erbium atoms return to the ground state via spontaneous emission. When the photons of the light of the signal to be amplified are directed to the erbium atom excited by the photons of the pump laser, the erbium atom emits a photon corresponding to the photon of the signal light.
As previously indicated, a side-effect of the amplification of an optical signal using an EDFA is the spontaneous emission of photons, which are in turn amplified, adding to the noise. The resulting spurious signal is known as Amplified Spontaneous Emission (“ASE”).
As an OFA amplifies a signal by a given factor, without adding information to the signal, signaling associated with supervision and control for the amplifier generally has to be carried out separately. One technique for accomplishing this has been to transmit supervisory messages by modulating the control signal of a pump laser unit in such a manner that the pumping light contains a supervisory signal to be transmitted. This technique suffers from the disadvantage that it requires two separate optical systems to be employed—one for receiving a payload signal and one for receiving the supervisory signal. Additionally, it can only be used in connection with frequencies that have a period longer than the lifetime of the fluorescence state.
Another technique has been to modulate a separate optical wavelength (e.g. 1510 nm or 1565 nm) to carry the supervisory and control information. This technique is relatively costly and sometimes requires wavelength converters to shift the supervisory channel from one part of the spectrum to another in cases in which Raman amplifiers or L/C band couplers are used. Yet another technique has been to use the Forward Error Correction (“FEC”) overhead bytes to carry the supervisory signal. This technique require access to the FEC framer at each side, which may present problems.